The present invention relates to plasma arc torches and, more particularly, to a method of forming an electrode for supporting an electric arc in a plasma arc torch.
Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treatment, melting, and annealing. Such torches include an electrode that supports an arc that extends from the electrode to the workpiece in a transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex flow of gas, and in some torch designs it is conventional to also envelop the gas and arc with a swirling jet of water.
The electrode used in conventional torches of the described type typically comprises an elongate tubular member composed of a material of high thermal conductivity, such as copper or a copper alloy. One conventional copper alloy includes 0.5% of tellerium (tellerium has a melting temperature of 841xc2x0 F.) to provide better machinability than pure copper. The forward or discharge end of the tubular electrode, known as a xe2x80x9cholderxe2x80x9d, includes a bottom end wall having an emissive element embedded therein which supports the arc. The emissive element is composed of a material that has a relatively low work function, which is defined in the art as the potential step, measured in electron volts (ev), which permits thermionic emission from the surface of a metal at a given temperature. In view of its low work function, the emissive element is thus capable of readily emitting electrons when an electrical potential is applied thereto. Commonly used emissive materials include hafnium, zirconium, tungsten, and their alloys.
Some electrodes include a relatively non-emissive member or xe2x80x9cseparatorxe2x80x9d, which is disposed about the emissive element and acts to prevent the arc from migrating from the emissive element to the copper holder. These non-emissive members are discussed in U.S. Pat. No. 5,023,425 to Severance, which is incorporated herein by reference. The thermal conductivity of electrodes is important for removing heat generated by the arc, which increases the usable life of the electrode. As such, the non-emissive member is also preferably formed from a highly thermally conductive metal, such as silver or silver alloys.
Many conventional electrodes are assembled by pressing the emissive insert into the metallic holder, or by pressing the emissive insert into the non-emissive member which is then pressed into the metallic holder. The interfaces between the press-fit emissive element, non-emissive member, and holder can negatively affect the thermal conductivity of the assembled electrode by creating a xe2x80x9cstepxe2x80x9d in the thermal conductivity at the interface of adjoining parts. This is especially true where the adjoining surfaces do not fit together very closely. Brazing is sometimes used to ensure sufficient thermal and electrical conduction. However, the use of brazing materials adds additional steps to the manufacture of an electrode, and brazing materials typically have a low melting point, which is disadvantageous when attempting to bond to the emissive element, as discussed below.
In order to help thermal conduction over the interfaces of the emissive element, non-emissive member, and holder, the assignee of the present invention has developed a diffusion bonding technique described in a co-pending application with Ser. No. 09/773,847 (xe2x80x9cthe ""847 applicationxe2x80x9d) entitled xe2x80x9cElectrode Diffusion Bondingxe2x80x9d, which is incorporated herein by reference. In the co-pending ""847 application, a post-assembly heating step is described that creates a diffusion bond between the non-emissive member and the metallic holder. The diffusion bond softens or smoothes the thermal interface between the two materials, while increasing the bond strength therebetween. As a result, the electrode has a longer operational life.
In the co-pending patent application Ser. No. 09/871,071 (xe2x80x9cthe ""071 applicationxe2x80x9d), which is also incorporated herein by reference, the assignee of the present invention has discovered that it is also sometimes desirable to improve the bond between the emissive element and non-emissive member by heating. The post-assembly heating step of the co-pending ""847 application is particularly advantageous for improving the bond between materials such as silver (in the case of the non-emissive member) and copper (in the case of the holder), but the relatively high temperature resistance of the emissive element (which is typically hafnium) may cause the bond between the non-emissive member and the holder to be destroyed if any heat treatment of the emissive element was attempted. As set forth in the ""071 application, a two stage assembly and heating process is provided wherein strong bonds are formed between the emissive element and non-emissive member and between the non-emissive member and metallic holder.
In particular, an emissive element, such as hafnium, is positioned in a non-emissive member, such as silver, and is heated to a temperature of between about 1700xc2x0 F. and 1800xc2x0 F. such that an intermetallic compound is formed between the hafnium and silver, thereby creating a strong and conductive bond. Thereafter, the emissive element and non-emissive member are bonded to a holder, such as copper, by way of a heating step that forms a eutectic alloy between the copper holder and the silver member. This heating step typically occurs between about 1400xc2x0 F. and 1450xc2x0 F. In particular, when copper and silver are heated together, a eutectic melting point is achieved (which is lower than the melting point of both pure silver and pure copper) at about 1432xc2x0 F. This second heating process forms a strong and conductive thermal bond between the holder and the non-emissive member such that the resulting electrode includes thermal bonds between both the hafnium emissive element and the silver non-emissive member, and between the silver non-emissive member and the copper holder. Such an arrangement greatly enhances the thermal conductivity of the electrode by bonding the base materials of the components, which allows heat to be readily removed from the arc emitting element and thereby enhances the operational life of the electrode.
However, with the method of the ""071 application, the heating steps for forming thermal bonds between the emissive element and the non-emissive member, and between the non-emissive member and the holder are conducted separately. In other words, the relatively low eutectic melting point between a silver member and a copper holder prevents heating to the much higher temperature that is necessary to form thermal bonds between the emissive element and the non-emissive member. The eutectic alloy formed between the silver member and the copper holder will simply melt away or evaporate if raised to a suitable hafnium/silver bonding temperature, leaving voids between the two members and preventing adequate thermal conduction.
In addition, the eutectic reaction that occurs between silver and copper occurs very rapidly at the eutectic temperature. Thus, if the heating process goes beyond the eutectic temperature for even a short period of time, the silver and copper can quickly intermix and destroy the other advantageous properties of those materials, such as the non-emissivity of silver. On a commercial production basis, the tight temperature tolerances can be difficult to achieve and consistent manufacture is challenging.
Thus, separate heating steps, as presented in an embodiment of the invention of the ""071 application, cause expense and delay in manufacturing costs that would desirably be avoided. In addition, the copper/silver eutectic reaction can be difficult to control on a commercial scale. Thus, there is a need in the industry for an electrode of the general type discussed above wherein only one heating step is required to form thermal bonding between the non-emissive member and the holder and, if desired, between the emissive element and the non-emissive member. In addition, there is a need for a method of commercial manufacture that easily accommodates thermal bonding between the non-emissive separator and the holder.
The present invention meets these objectives and others by the use of a third metal, such as nickel, at the interface of the copper holder and silver non-emissive member. In a particular embodiment, the copper of the holder is alloyed.with nickel, which attenuates the eutectic reaction between the silver and the copper. The nickel causes the eutectic reaction to be slowed such that a thermal bond can be formed between the holder and the non-emissive member at a higher temperature than the eutectic temperature of pure silver and pure copper. This bond can be formed over a greater temperature range and during a higher-temperature heating step that can be used also to form thermal bonding between the hafnium emissive element and the silver non-emissive member. As a result, electrodes according to the present invention can advantageously be formed with bonding both between the non-emissive member and the holder and between the emissive element and the non-emissive member during only one heating cycle.
The third metal can be alloyed in the metallic holder and/or can also be alloyed in the metal of the non-emissive member. A preferred composition is about 10% nickel by weight of the metallic holder with the remainder comprising copper. However, it is not necessary for the third metal to be alloyed, and either of the adjoining components can instead be plated. In addition, the third metal can be presented in powdered form between the non-emissive member and the emissive element, or by way of a thin sleeve that surrounds the non-emissive member and separates the non-emissive member from the holder. In addition, it is not necessary that the third metal comprises nickel and it may comprise at least one of the group consisting of zinc, iron, cobalt and chromium. The first metal may also comprise sterling silver.
Thus, the present invention provides electrodes and methods of making electrodes having stronger bonds between the elements thereof, which improves the strength and operational life span of the electrodes. In particular, these electrodes can be manufactured inexpensively and relatively quickly with only a single heating step. Furthermore, the methods of making electrodes according to the present invention allow the formation of electrodes that do not require brazing materials between the emissive element, non-emissive member, or metallic holder.